ACYL RADICAL-MEDIATED POLYENE CYCLIZATIONS DIRECTED TOWARDS STEROID RING SYNTHESIS

Treatment of appropriately substituted Se-phenyl 5,9,13-triene- and 5, 9,13,17-tetraene-selenoates, i.e 9b, 33, 40, 47a and 47b, with Bu,3SnH -AIBN is found to lead to angular six-ring fused polycycles, viz. 20, 34, 50, 53 and 54 respectively, via consecutive 6-endo-trig modes of c yclisations, starting from the corresponding polyene acyl radical inte rmediates. The structures and stereochemistries of the polycyclic prod ucts were determined largely from detailed analysis and correlation of C-13 NMR spectroscopic data. The trans-anti-trans stereochemistry of the tricyclic ketone 50 was established from X-ray analysis of a solid solution of a 1:1 mixture of ring C methyl epimers of the correspondi ng 2,4-dinitrophenylhydrazone derivative.